虽然没有文档说明，但Collection.FindOne()暗示了Limit=1的行为，这是常识。Collection.FindOne()的返回值不给予访问多个结果文档，这就是为什么options.FindOne甚至没有SetLimit()方法。
如果你检查源代码，它在那里：

// Unconditionally send a limit to make sure only one document is returned and the cursor is not kept open
// by the server.
findOpts = append(findOpts, options.Find().SetLimit(-1))

请注意，FindOptions.Limit记录了以下内容：

// Limit is the maximum number of documents to return. The default value is 0, which means that all documents matching the
// filter will be returned. A negative limit specifies that the resulting documents should be returned in a single
// batch. The default value is 0.
Limit *int64